In conventional impulse turbines the moving blades are molted on disks that are seared to the rotor.
For reasons relating to the dynamic behavior of the rotor (see FIG. 1), it is essential to give the rotor sufficient stiffness, thus retiring a hub of adequate diameter DR. In contrast, for reasons of efficiency, it is necessary to keep this diameter as small as possible: reducing the diameter serves to reduce the leakage section between the stationary and moving portions, thereby reducing leakage itself. These two rules enable the rotor diameter DR to be determined as a compromise.
In order to be able to fix the moving blades on a rotor disk without the disk loosing its mechanical qualities, it is necessary to leave sufficient space between the base of a blade and the shaft of the rotor. Under such conditions, when the diameter DR is fixed, there exists a bottom limit on the blade base diameter DB.
In addition, to improve efficiency (see FIG. 2) it is advantageous to increase the slenderness ratio Z (the ratio between the tip diameter of the blade D.sub.s to the base diameter of the blade DB). The greater Z (for given blade width), the lower the losses due to secondary flows (e). However, once the operating conditions of the machine have been fixed, the steam flow section is determined. This section is proportional to DB.sup.2 (Z.sup.2 -1). It is therefore clear that to improve efficiency, DB should be reduced as much as possible, down to its lower limit.
In addition, it is necessary to reduce as much as possible all leakage between the stationary portions and the moving portions (f, f') in order to increase efficiency.
In particular, steam leakage (f') passing beneath the stationary blading gives rise to a large efficiency penalty: not only does this steam provide no useful work, but because of the presence of disks downstream from the stationary blading, it also penetrates radially into the main flow and degrades it.
Finally, in some cases, the relatively low pressure forces (p) on the rotor and on the moving blading may give rise to a balancing piston of diameter slightly greater than DR being provided at the head of the machine, thereby also contributing to a small drop in efficiency.
There also exist "reaction" steam turbines in which the pressure drop across a stage is split into two substantially equal portions between the stationary blading and the moving blading of the stage.
The moving blades are fixed directly on the rotor shaft. The base diameter of the blades DB is close to the shaft diameter DR (drum rotor).
Because of the pressure drop across the moving blading, the thrust on said blading is very high and necessarily gives rise to a balancing piston of large diameter being provided at the head of the machine to balance said thrust, with the diameter of the piston possibly being as great at DS (moving blade tip diameter). This contributes greatly to reducing efficiency.
It can thus be seen that thrust problems penalize the efficiency of reaction turbines greatly because of the leakage due to the large diameter of the balancing piston.
It is not possible in reaction turbines to reduce leakage by reducing the base diameter DB.
Such an operation would lead to an increase in the number of stages that would be unacceptable from the point of view of cost and from the point of view of the longitudinal size of the module under consideration, particularly if it is a high-pressure module.
Finally, it should be observed that in reaction machines, i.e. drum rotor machines, the leakage passing beneath the stationary blades does not have the same disturbing effect as it has in impulse machines with disks.